Air brake systems for a vehicle such as a bus, truck, trailer and other heavy-duty vehicles or the like typically include a brake shoe and drum assembly which is actuated by means of an actuator assembly operated by the selective application of compressed air. Conventional air brake actuators have both a service brake actuator for actuating the brakes under normal driving conditions by the application of compressed air and a spring-type emergency brake actuator which causes actuation of the brakes when air pressure has been released. The emergency brake actuator includes a strong compression spring which forces application of the brake when air is released. This is often referred to as the spring brake.
Air-operated brake actuators are either piston type or diaphragm type. In the diaphragm type spring brake actuator, two air-operated diaphragm brake actuators are typically arranged in a tandem configuration, which includes an air-operated service brake actuator for applying the normal operating brakes of the vehicle, and a spring brake actuator for applying the parking or emergency brakes of the vehicle. Both the service brake actuator and the spring brake actuator include a housing having an elastomeric diaphragm dividing the interior of the housing into two distinct fluid chambers. On the other hand, the piston brake actuator operates under basically the same principles as above described, except that instead of a diaphragm, a piston with a sliding seal at the outside diameter reciprocates in a cylinder for applying the normal and/or parking brakes of the vehicles.
In a typical service brake actuator, the service brake section is divided into a pressure chamber and a pushrod chamber. The pressure chamber is fluidly connected to a source of pressurized air and the pushrod chamber mounts a pushrod, which is coupled to the brake assembly, whereby the introduction and exhaustion of pressurized air into the pressurized chamber reciprocates the pushrod into and out of the actuator to apply and release the operating brakes.
In a typical spring brake actuator, the spring brake section is divided into a pressure chamber and a spring chamber. A spring piston is positioned in the spring chamber between the diaphragm and a strong compression spring, whose opposing end abuts the housing. In one well-known configuration, an actuator rod extends from the spring piston, through the diaphragm, into the pressure chamber, and through a dividing wall separating the spring brake actuator from the service brake actuator. The end of the actuator is fluidly connected to the pressure chamber of the service brake actuator.
When applying the parking brakes, the spring brake actuator pressure is discharged from the pressure chamber and the large force compression spring pushes the spring piston and the diaphragm toward the dividing wall between the spring brake actuator and the service brake actuator. In this position, the actuator rod connected to the spring piston is pushed forward extending into the service section through the dividing center wall applying the parking or emergency brakes and thus forcing the vehicle to stop or remain parked. To release the parking brake, the pressure chamber is closed to the atmosphere and pressurized air is introduced into the pressure chamber of the spring brake actuator which expands the pressure chamber, moving the diaphragm and spring piston toward the opposing end of the spring brake actuator housing, thereby compressing the strong compression spring.
One known problem in association with spring brake actuators of this design is that as the large force compression spring is compressed, the pressure chamber increases in volume and the spring chamber decreases in volume, resulting in a pressure increase in the spring chamber unless it includes a particular system for relieving the pressure increase in the spring chamber. The build-up of pressure in the spring chamber upon the release of the brake is highly undesirable in that any pressure build-up in the spring chamber must be offset by an increased pressure in the pressure chamber in order to fully compress the spring and thus fully releasing the brake.
The pressure build-up in the spring chamber is exacerbated in that most pressurized air systems for heavy-duty vehicles operate at an industry standard maximum pressure. The combined force of the spring and the increase in air pressure in the spring chamber cannot approach the maximum for the brake to operate properly. As the combined force associated with the pressure of the spring and the build-up of pressure in the spring chamber approach the force applied by the maximum pressure, the brake can fail to release, only partially release, or release very slowly, all of which are undesirable.
Prior art designs have used a pushrod assembly such that when air pressure is applied to the diaphragm, the center pushrod is moved to the retracted position. One such design consists of a center pushrod and a separate attached plate on the opposite side of the spring section diaphragm from the spring piston. In this instance the center pushrod and separate plate are not attached to the spring piston and require the use of a return spring to assure the center pushrod is restrained in the brake release position when air pressure is applied to a flexible diaphragm forcing the parking spring to compress. Another similar existing design consists of a center pushrod and separate attached plate on the spring piston side of the diaphragm with the center rod projecting through the diaphragm. This design employs a fixed attachment of the diaphragm to the center rod and/or separate plate. This design does not require a return spring as the attached diaphragm will cause the center rod to move to the retracted position.
Another such design involves use of a solid attachment of the spring brake diaphragm to the center pushrod assembly and/or to the parking spring piston and center rod, so that when air pressure is applied to a flexible diaphragm, the parking spring is compressed. Both of the designs with a fixed attachment of the diaphragm and center rod, separate plate or spring piston, however, restrict the rotational and translational movement of the diaphragm with respect to the center pushrod. As a result of restricting translational and rotational movement, high stress is put upon the diaphragm at the attachment point of the diaphragm to the center pushrod. This is undesirable as increased stress on the diaphragm can cause it to malfunction or even crack.
It is desirable to develop a system that reduces stress at the diaphragm interface as this preserves the integrity of the diaphragm and provides longevity to the brake system. Furthermore, it is advantageous to eliminate the need for a center rod return spring in brake assemblies with center pushrod and fixed plates which are separate from the parking spring piston, as one of the embodiments of the present invention portrays.